In addition to what has been said, there is but little for me to offer
on the subject of walls beyond the general question of stability. In
mill construction, walls of uniform thickness have been displaced by
pilastered walls, about sixteen inches thick at the upper story, and
increasing four inches in thickness with each story below.
The remainder of the walls is from four to six inches less in
thickness than at the pilasters. Frequently the outside dimensions of
these pilasters are somewhat increased, giving greater stability and
artistic effect. By leaving hollow flues within them, and using these
flues as conductors for heated air which may be forced in by a blower,
such pilasters afford a means for the most efficient method of warming
the building.
Consideration must be given to the contraction of brick masonry,
especially when an extension or addition is to be made to an older
building. This shrinkage amounts to about three-sixteenths of an inch
to the rod, an item which is of considerable importance in the floors
of high buildings, where the aggregate difference is very appreciable.
Some degree of annoyance is caused by neglect to consider this element
of shrinkage in reference to the window and door frames, which should
have a slight space above them allowing for such contraction. This
contraction is often the source of serious trouble in brick buildings
with stone faces, the shrinkage of the brick imposing excessive stress
on the stone. Instances of this are quite frequent, especially in
large public buildings, notably the capitol at Hartford and the public
building at Philadelphia, where the shivering of the joints of the
stone work gave undue alarm, on the general assumption that it
indicated a dangerous structural weakness. The difficulty has, I
believe, been entirely remedied in both cases.
The limit of good practice on loads upon brickwork is eight to ten
tons per square foot, although it is true that these loads are largely
exceeded at times. It is not to be shown, however, that the limits of
safety in regard to desirable construction should be confined to the
use of masonry for any low buildings. Structures which may be said to
be equal to those of brickwork, as far as commercial risk is
concerned, can be built wholly or in part of wood so as to conform to
all practical conditions of safety. This statement does not apply
except to low buildings of one or possibly two stories in height,
where the timber cannot be subjected to the intense blast of flame
occurring when a high building is on fire.
Mr. George H. Corliss, the eminent engine builder, of Providence,
first built a one-story machine shop, with brick walls extending only
to the base of the windows, above this the windows being very close
together, with solid timber construction between them.
Another method is to place upright posts reaching from the sill to the
roof timbers, and to lay three-inch plank on the outside of such posts
up to the line of the windows. A sheathing on the outside plank
between the timbers is laid vertically and fastened to horizontal
furring strips. In some instances a small amount of mortar is placed
over each of the furring strips. The reason for this arrangement is to
prevent the formation of vertical flues, which are such a potent
factor in the extension of fires.